Accordingly, the invention relates to hybrid bearings. Hybrid bearings are rolling bearings which have bearing rings, or at least the raceways of the bearing rings, made from steel and which are provided with rolling bodies made from ceramic, or in which at least one bearing ring is made from ceramic and the rolling bodies are made from steel.
The materials used for the ceramic rolling bearing element are all industrial ceramics, in particular silicon nitride (Si3N4), but also silicon carbides and also aluminum and zinc oxides. The use, construction and advantages and disadvantages of using hybrid bearings are extensively described in the publication by FAG Kugelfischer Georg Schäfer KGaA, Publ. No. WL 40 204 DA 80/11 from 1990 “Hochleistungskeramik in FAG Wälzlagern” [High-performance ceramics in FAG roller bearings].
Excellent properties of ceramic for use as roller bearing material for roller bearing parts which are subject to high and extremely high loads include properties such as low weight, low thermal expansion, high hardness and heat resistance, good dimensional stability at extremely high temperatures, high chemical resistance and high corrosion resistance, high modulus of elasticity, lower frictional torque at high rotational speeds and lower heat production, lack of magnetism and insulator properties.
The centrifugal force of the rolling bodies should also be taken into account when determining the loads on fast-moving roller bearings. At very high rotational speeds, such as for example for applications in driving mechanisms, the centrifugal forces are generally even dominant over the bearing loads. On account of the low weight of ceramic, at high rotational speeds the centrifugal forces generated are lower at rolling bodies made from ceramic. The density of ceramic, for example silicon nitride, is only approx. 40% of the density of steel.
Ceramic materials have a significantly higher modulus of elasticity, for example 1.5 times, than steel. Consequently, for the same loading, the specific stress in the rolling contact is higher than with contact parts made from steel, since the pressure ellipse produced by the ceramic rolling bodies in the rolling contact is smaller. In general, therefore, rolling pairings in which both components are made from ceramic, and in particular rolling pairings in which one rolling part is made from steel and the other is made from ceramic, are not able to withstand as high a load as rolling pairings in which both contact parts are made from steel.
Components made from ceramic have a fundamentally different failure mechanism than components made from roller bearing steel. On account of the brittleness of the ceramic material, in the event of overloading, ceramic fractures without any significant plastic deformation. This property has caused the person skilled in the art to consider that in the event of damage to components made from ceramic, such as small pieces of the surface breaking off, in a roller bearing, the ceramic components will be the first to suffer extensive damage, and the steel components will only undergo such damage at a later stage, as a secondary phenomenon. However, as has emerged and as has also been described in the prior art cited above, under certain circumstances ceramic components which have suffered preliminary damage as a result of impurities and inclusions and pores, cracking nuclei and microcracks and overloading and/or as a result of foreign particles in the rolling contact, have initially proven to still have a long service life. The abrasion of the ceramic and/or particles which have broken out of the surface of the ceramic component in some cases act as an abrasive on the raceways made from steel or pass into the rolling contact, where they first of all damage outer layers of the rolling parts made from steel.
The sensitivity of the surfaces of the roller bearing parts made from steel is dependent on the nature and magnitude of the stresses prevailing at and below the surface (in the outer layer). In this context, the term nature is to be understood as meaning residual stresses (tensile or compressive stresses) in the outer layer of the component or stresses acting on the component as a result of external action. Tensile stresses at the surface and in the outer layer below the surface increase the sensitivity of the component. Tensile stresses of this type are caused on the one hand by the pretreatment (heat treatment and hard machining) of the component and by operating conditions to which the component is exposed. For example, at the outer raceway of an inner ring and at the edges, the residual tensile stresses are further intensified by tensile stresses from the required press fit of the inner ring, for example on a shaft. If this inner ring additionally rotates at high speeds, the tensile stresses may reach a level at which the susceptibility of the inner ring to failure is significantly increased. Locally high contact pressures caused by particles which have broken off from the ceramic components and entered the rolling contact may damage the outer layer of the raceways in such a way that microcracks are formed. These microcracks then propagate to produce pitting and further, extensive damage.
The hard and sharp edges of the possible break-out locations on the rolling part made from ceramic have similar effects on the rolling parts made from steel. These break-out locations cause the steel surface to be highly stressed as a result of abrasive wear. As a result, at high rotational speeds and loads, the rate at which damage progresses may be drastically increased.
The use of ceramic components having the materials problems described above has as far as possible been avoided by suitable testing, enabling defective parts of this type to be scrapped. However, the rolling contact is endangered, in particular in bearings which are greatly influenced by the surroundings, by foreign particles which pass into the rolling contact, for example together with the lubricant. Foreign particles of this type then likewise first of all cause destruction of rolling parts made from steel, in accordance with the mechanisms outlined above.